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Earthquake sensors called seismometers stationed i... n the Caribbean detected the magnitude 7 earthquake that struck near the Haitian capital of Port-au-Prince. The signal was relayed to scientists, including those at the U.S. Geological Survey Center in Reston.

"At 21 minutes, they knew about 2 million people would be affected by the shaking," said scientist David Applegate, who studies earthquakes and other geologic hazards at the U.S. Geological Survey.

The report sent by Applegate and others would help rescue and aid workers to predict and later define the scope of Haiti's damage, death tolls and needs.

"In a lot of earthquakes, places like CNN know about it fast. But in a situation like Haiti, the communication isn't there," scientist John Filson said.

Using information generated by seismometers in the Caribbean, the scientists could plot on a map where the most damage likely was caused and prepare officials and rescuers for the aftershocks, or smaller earthquakes, that would follow the Jan. 12 event.

"One of the things that makes earthquakes so difficult to deal with is the aftershocks," Applegate said. "Buildings that withstood the first quake might crumble under the next."

He said aftershocks "are very damaging psychologically."

After the initial quake, the USGS, a branch of the Interior Department, predicted a 25 percent chance of a magnitude 6 aftershock and a 90 percent chance of a magnitude 5 or higher aftershock.

More than 50 aftershocks of magnitude 4.5 or higher have been recorded in Haiti since Jan. 12.

"A way to think about it is to picture ripples on a pond," said scientist Michael L. Blanpied, a Reston resident.

"Magnitude is the rock you throw into the pond," Filson said.